Published studies indicate that when adrenal tissue in vitro was treated with ACTH the increase in steroidogenesis was associated with a release of prostaglandin E2 (PGE2) into the medium. Other studies indicate that treatment of adrenal tissue in vitro with PGE2 stimulates the formation of aldosterone and cortisol. Such results suggest that the adrenal forms PGE2 and that PGE2 can stimulate steroidogenesis. To explore further the role of prostaglandins in steroidogenesis in man, patients with primary aldosteronism, who have very low values for plasma renin activity and high values for aldosterone in blood and urine were treated with a prostaglandin synthetase inhibitor. There was no effect of prostaglandin inhibition on the production of aldosterone by the adrenal adenoma or on the production of 17-hydroxycorticosteroids and 17-ketosteroids by the unaffected adrenal tissue.